AWS Load Balancer Terraform module (https://github.com/terraform-aws-modules/terraform-aws-alb): target_group_index = 0, what is it?

Question

I try to use the module, AWS Application and Network Load Balancer (ALB & NLB) Terraform module, https://github.com/terraform-aws-modules/terraform-aws-alb. There is a sample usage of Application Load Balancer, see below:

module "alb" {
  source  = "terraform-aws-modules/alb/aws"
  version = "~> 5.0"
  
  name = "my-alb"

  load_balancer_type = "application"

  vpc_id             = "vpc-abcde012"
  subnets            = ["subnet-abcde012", "subnet-bcde012a"]
  security_groups    = ["sg-edcd9784", "sg-edcd9785"]
  
  access_logs = {
    bucket = "my-alb-logs"
  }

  target_groups = [
    {
      name_prefix      = "pref-"
      backend_protocol = "HTTP"
      backend_port     = 80
      target_type      = "instance"
    }
  ]

  https_listeners = [
    {
      port               = 443
      protocol           = "HTTPS"
      certificate_arn    = "arn:aws:iam::123456789012:server-certificate/test_cert-123456789012"
      target_group_index = 0
    }
  ]

  http_tcp_listeners = [
    {
      port               = 80
      protocol           = "HTTP"
      target_group_index = 0
    }
  ]

  tags = {
    Environment = "Test"
  }
}

I understand the target_groups is an array. But, why target_group_index = 0 not target_group_index = 1 or target_group_index = 2? What is the index 0?

Answer 1

In the example you've posted, there is only one target group (TG):

  target_groups = [
    {
      name_prefix      = "pref-"
      backend_protocol = "HTTP"
      backend_port     = 80
      target_type      = "instance"
    }
  ]

Thus the listeners use target_group_index = 0 to indicate that they apply to this one specific TG. If you have more TGs, you would use target_group_index = 1, 2, 3 ... to specify which listener applies to each TG.